1. Field of the Invention
The invention relates generally to thermal transfer structures; and more particularly to an operating fluid and a heat pipe containing such operating fluid.
2. Description of Prior Art
Electronic components such as semiconductor chips are becoming progressively smaller, while at the same time heat dissipation requirements thereof are increasing. In many contemporary applications, a heat pipe is one of the most efficient systems in use for transferring heat.
Referring to FIG. 3, a typical heat pipe 10 is a vessel that comprises a pipe 11, a wick 12 and a precise amount of liquid operating fluid 13. The wick 12 is fixedly engaged with an inside wall (not labeled) of the pipe 11. The liquid operating fluid 13 is sealed in the pipe 11 and soaks the wick 12. One end of the heat pipe is an evaporator section, and the other end of the heat pipe is a condenser section. The evaporator section is disposed in thermal communication with an external heat source, while the condenser section is disposed in thermal communication with an external heat sink. Further, an adiabatic section can be adopted to connect the evaporator section to the condenser section, with heat being transferred within the heat pipe from the evaporator section to the condenser section through the adiabatic section.
An operating principle of the heat pipe 10 is as follows. Liquid operating fluid 13 is located in the evaporator section of the heat pipe 10. A heat source such as ambient hot air transfers heat 15 by conduction through the wall of the heat pipe 10 to the liquid operating fluid 13. The temperature of the liquid operating fluid 13 rises steadily commensurate with the provision of the heat 15. This temperature rise continues up to a temperature at which the liquid operating fluid 13 changes from the liquid state to a vapor state. At this vaporization temperature, the provision of additional heat 15 transforms the liquid operating fluid 13 into vaporized operating fluid 14. Vapor pressure drives the vaporized operating fluid 14 through the adiabatic section to the condenser section of the heat pipe 10. At the condenser section, the vaporized operating fluid 14 transfers the heat 15 absorbed in the evaporator section to a heat sink (not shown) located at the condenser section of the heat pipe 10, thereby transforming the vaporized operating fluid 14 back into liquid operating fluid 13. Capillary action and/or gravity return the liquid operating fluid 13 back to the evaporator section. The heat pipe 10 continues the process of transferring heat 15 as long as there is a temperature differential between the evaporator section and the condenser section of the heat pipe 10 and as long as the heat 15 is sufficient to vaporize the liquid operating fluid 13 at the evaporator section.
In order to ensure the effective operation of the heat pipe 10, the operating fluid 13 must has high thermal conductivity. Conventional heat pipes generally adopt pure liquids to act as the operating fluids. U.S. Pat. No. 6,407,922 discloses such kind of heat pipe. The heat pipe comprises a precise amount of operating fluid. The operating fluid is selected from the group consisting of pure alcohol, freon, water and acetone. However, for many applications, the thermal conductivities of these operating fluids are too low. The rate of heat transfer is too slow, and the operating efficiency of the heat pipe is unsatisfactory.
In order to enhance the thermal conductivity of pure operating fluids, metal salts or metal compounds are mixed into them. China Pat. No. 98110556.4 discloses one such kind of operating fluid. The operating fluid comprises 1000 g (grams) of pure water, and 30-50 g of potassium dichromate, 10-15 g of sodium perborate, 3-5 g of boracic acid, 1-3 g of sodium peroxide, 0.5-1.5 g of aluminum hydroxide, 0.2-0.5 g of dicobalt trioxide and 0.2-0.5 g of manganese dioxide mixed in the pure water. The thermal conductivity of the operating fluid is higher than that of pure water.
However, because the operating fluid comprises metal salts or metal compounds, during operation of the heat pipe the metal salts or metal compounds easily aggregate when the operating fluid evaporates. This reduces the thermal conductivity of the operating fluid. In addition, if the heat pipe is damaged or becomes worn out, the operating fluid is generally disposed of without recycling. This can lead to pollution of the environment. On the other hand, the cost of recycling the operating fluid is very high. Furthermore, because the operating fluid is water-based, and it can be applied only in certain kinds of heat pipes.
A new operating fluid for a heat pipe which overcomes the above-mentioned problems is desired.